For the past fifty years proximity transducer systems based on the eddy current effect have dominated the measurement of vibration and shaft position in machines with fluid film bearings. These systems convert the physical distance from a probe to a metal target into a voltage proportional to that physical distance. A proximity transducer system has by its nature a frequency response from DC (static distance) to about 10 Khz of AC or (dynamic distance). This allows these sensors to be useful for measuring static (DC) distances such as the position of a shaft relative to a thrust bearing and the dynamic AC movement of a machine shaft (e.g., vibration). Proximity transducer systems are also used for measuring the dynamic position of rods, pistons or other and mechanically moving parts on reciprocating machines. When a proximity transducer system is mounted to a fixed part of a machine observing the rotating shaft the AC component of the output is proportional to vibration of the shaft relative to the machine case or bearing. This direct vibration measurement has become the industry standard.
Typical proximity transducer systems consist of a probe tip located within a probe body, a separate electronics module, a monitoring system, and an interface cable connecting the electronics module to the monitoring system. The probe tip typically contains a coil of wire that is located within the probe body and arranged to be placed in close proximity of the component to be observed (e.g., a machine shaft or thrust collar). The probe body not only supports the sensing tip, but also allows setting the static distance from the tip to the target. The material making up the target has to be metal for the eddy current effect to be realized. A coaxial cable may be provided for connecting the probe tip's wire coil to the electronics module when a separate electronics module is used. If the probe is an integral one, i.e., the probe contains the electronics, a separate electronics module is unnecessary, as is a coaxial cable. In any case the electronics module contains electronics for driving (powering) the probe tip and for converting the output signal from the probe, i.e., the measured distance, to a linear voltage signal which is represented in volts/distance units. The interface cable serves to connect the proximity electronics module to the monitoring system is typically a three-wire twisted shielded cable. The monitoring system can take various forms, e.g., it may be designed to protect machines, provide current values, alarms, diagnostic information, or many other uses. In all case, the monitoring system provides power to the proximity transducer system and accepts the signal from the proximity transducer system. This signal is then analyzed for various useful attributes such as overall vibration, vibration waveforms, vibration spectrums, vibration phase and amplitude, thrust position, compressor rod position, compressor piston position and so forth. The monitoring system may be custom made, built of existing systems such as PLCs, machine unit controllers, computer DAQ functions or any number of realizations.
As should be appreciated by those skilled in the art, the three-wire cables used in the foregoing proximity transducer systems have a number of drawbacks. For example, each channel requires a shielded three-wire twisted cable. Three-wire cables are not as common as shielded two wire cables and are more expensive. Moreover, three wire cables exhibit significant cable bulk, requiring a larger conduit. Further still, the voltage interface used in the current interface is typically terminated with a 10K ohm load resistor. This makes the internal signals, power, signal and common, susceptible to conducted EMI. Unwanted conducted currents entering the system will generate voltage across a relatively large load resistor. Because the interface is not differential, this voltage can create error signals that can cause significant performance issues with the monitoring system up to and including creation of false alarms. Furthermore, in potentially explosive or hazardous applications where proximity transducer systems are used, such systems commonly incorporate the use of a zener diode as a safety barrier between the monitoring system and the proximity electronics module. Such an arrangement with conventional three-wire voltage based systems results in a reduced linear range and a decreased scale factor.
Thus, a need exists for a cable connection between the probe electronics module (connected by coax or integral) and the monitoring system (or equivalent) which overcomes those disadvantages.
The subject invention addresses that need. To that end, this invention targets the electrical design and properties of the cable connection and reduces the number of connection wires from three to two. In addition it changes the mode of the analog interface from single ended voltage to a current loop, which provides both the dynamic signal transmission and power for the proximity transducer.